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We now know how light can act as a wave or a particle, depending on the situation. You might wonder, though, why a chemistry textbook would waste a whole lesson on light. Light, like matter, is part of the universe, but chemists aren't responsible for studying the entire universe. Chemists are responsible for studying chemicals. What does light have to do with chemicals? Why do chemists need to know about light?

It turns out that scientists can actually learn a lot about chemicals by observing how they interact with light. Different chemicals behave differently when struck with a beam of light. In fact, the same chemical will interact differently with differently colored beams of light. To understand what light can tell us about different chemicals, though, we must first look at the electromagnetic spectrum a little more carefully.

If you have a class that requires a lot of work, you might find yourself saying something like, "I'm continuously doing homework for this class". Think about what you mean by that. You probably mean that the homework seems to be non-stop. Every time you finish one assignment, the teacher gives another, so you never get a break. When scientists use the word continuous, it has a similar meaning. It means no gaps, no holes, and no breaks of any kind.

Scientists don't use the word continuous to describe homework, but they do use it to describe electromagnetic spectra (spectra is just the plural word for spectrum). In the last section, you learned that an electromagnetic spectrum was a list of light arranged in order of increasing wavelength. A continuous electromagnetic spectrum, then, includes every possible wavelength of light between the wavelength at the beginning of the list and the wavelength at the end. If you find that definition confusing, consider the following example.

Suppose you have a continuous spectrum that begins with light at a wavelength of 500 nm, and ends with light at a wavelength of 600 nm. Because it's continuous, that spectrum contains light with any wavelength between 500 nm and 600 nm. It contains light with a wavelength of 550 nm. It contains light with a wavelength of 545 nm. It contains light with a wavelength of 567.3 nm. It even contains light with a wavelength of 599.99999 nm. Write down any number (including a number with decimal places) that is bigger than 500 and smaller than 600. A continuous electromagnetic spectrum between 500 nm and 600 nm will include light with a wavelength equal to the number you've written down.

As shown in the last section, within the visible range of the electromagnetic spectrum, a light's wavelength corresponds to its color. Therefore, another way of defining a continuous spectrum in the visible range is to say that it is a spectrum which contains every possible color between the color at the beginning of the list and the color at the end. Figure 5.13 shows several examples of continuous spectra in the visible light range. The first continuous spectrum starts with a deep indigo blue and ends with red. Notice how the colors in this spectrum change smoothly all the way from indigo to red. There are no gaps, or missing colors. The same is true of the second continuous spectrum. The second spectrum again starts with a deep indigo blue, but this time ends with yellow. Once more the colors in the spectrum change smoothly without any gaps or holes, which makes the spectrum continuous. The third spectrum is also continuous, only this time it starts with the color green and ends with the color orange.

Figure 5.13: Several examples of continuous spectra in the visible range.

Not all electromagnetic spectra are continuous. Sometimes they contain gaps or holes. Scientists call electromagnetic spectra that contain gaps or holes discontinuous. Let's reexamine spectra that start at 500 nm and end at 600 nm. Discontinuous spectra in this range will include light with some, but not all wavelengths of light greater than 500 nm and less than 600 nm. A discontinuous spectrum might, for example, only contain light with wavelengths of 500 nm, 523 nm and 600 nm. Obviously you can think of many numbers that lie between 500 and 600 which aren't included in that list (534 is one example). Therefore, the spectrum is discontinuous. A different discontinuous spectrum between 500 nm and 600 nm might contain every wavelength of light except 533 nm. In this case, almost every wavelength of light is included, but since 533 nm is missing, the spectrum is still discontinuous.

Figure 5.14 shows several examples of discontinuous spectra in the range of visible light. Again, since the wavelength of a beam of light corresponds to its color, you can clearly see when an electromagnetic spectrum in the visible range is discontinuous – there will be colors missing! In the first example, only a few shades of green are missing from the middle of the spectrum. Nevertheless, the missing shades of green make the spectrum discontinuous. The next two examples in Figure 5.14 have even bigger gaps of missing color, so it's even more obvious that they are discontinuous.

Figure 5.14: Several examples of discontinuous spectra in the visible range.

The concept of a continuous spectrum compared to a discontinuous spectrum may seem a little silly. So what if one spectrum contains every possible wavelength, while another skips wavelengths here and there! Why does that matter? To understand the importance of continuous and discontinuous spectra, we have to look a little more closely at the ways in which light interacts with matter, and how those interactions can actually produce electromagnetic spectra.

Light from the sun is a continuous spectrum. In other words, when you go to the beach and sunbathe, you are bombarded by light beams of every different wavelength in the electromagnetic spectrum Certain wavelengths bounce off your skin, while others interact in ways that lead to a tan or even sun burn. For example, you've probably seen sunscreens that offer UV protection. Ultraviolet light has wavelengths that are smaller than those of visible blue light. In addition to the white light that we see, the continuous spectrum of light from the sun contains UV light, and light with that range of wavelengths can be dangerous to human skin cells.

A sunbather trying to avoid tan lines and possibly too much UV radiation by having sunscreen applied to her back.

Of course, when you're lying in the sun on the beach, you don't actually see a rainbow shining down on you, do you? Instead, you see white light. As a result, you might be skeptical and find it hard to believe that sunlight forms a continuous spectrum. Surely, the beams of light coming from the sun don't contain every possible wavelength of light… surely, they only contain the wavelengths of light corresponding to the "color" white. That argument might seem logical to you, but you've fallen into a common trap – white isn't a color. If you take a careful look at the electromagnetic spectrum, what you'll notice is that there is no "white" light in the visible range. It turns out that white light does not come from light of any specific wavelength or range of wavelengths. Rather, in order for our eyes to see white, they must actually receive light of every wavelength in the entire visible spectrum.

When sunlight passes through water, the white sunlight is spread out so that you can actually see the entire spectrum of brightly colored light composing it. This is what we know as a rainbow.

A rainbow at the beach.

Electric current passed through neon gas creates an orange glow.

Electric current passed through argon gas creates a blue glow.

Electric current passed through helium gas creates a pink glow.

Electric current passed through xenon gas creates a purple glow.

White is not a color, and there is no light with a wavelength corresponding to "white". Instead, white light is formed when light of every wavelength in the visible spectrum is mixed together.

Remember that by passing white light through a prism, you are able to split the light into a rainbow, revealing all of the different colors, or different wavelengths of light that make up white light.

Now that we know where to find continuous spectra (light from the sun or any other source of pure white light), let's discuss where and when discontinuous spectra appear in our world. By passing an electric current or an electric spark through certain types of matter, it's possible to make that matter glow. Neon lights are one common example of this phenomenon. When an electric current travels through neon gas, the neon glows bright orange.

Neon isn't the only gas that lights up when electricity passes through it. Electricity causes argon to glow blue and helium to glow pink. In fact, electricity causes every element in the entire periodic table to glow with a distinct color. As you might have guessed, the light from a glowing sample of neon or argon is very different from the light shining down from the sun. Unlike sunlight, which is white, elements such as neon and argon glow in colors, which means that the light they emit (or send forth) is missing certain wavelengths – if it wasn't, it would appear white to our eyes.

Remember how sunlight spreads out into a rainbow, or a continuous spectrum, when you pass it through a prism? Well, when you pass light from a sample of glowing hydrogen through a prism, it doesn't spread out into a continuous spectrum. Instead, it spreads out into a discontinuous spectrum, with only four lines of colored light. A similar thing happens when you pass the light from a sample of glowing neon, or argon, or even sodium through a prism. Instead of getting a continuous spectrum, you get a discontinuous spectrum composed of a series of colored lines. The particular series of colored lines that you get out of any specific element is called the element's atomic spectrum or emission spectrum. Each element has an emission spectrum that is characteristic to that element. In other words, the emission spectrum from sodium is always the same and is different than the emission spectrum from any other element, like calcium or helium, or gold.

The emission spectrum for hydrogen.

The emission spectrum for iron.

Emission spectra are important to scientists for two reasons. First, because an element's emission spectrum is characteristic of the element, scientists can often use emission spectra to determine which elements are present or absent in an unknown sample. If the emission spectrum from the sample contains lines of light that correspond to sodium’s emission spectrum, then the sample contains sodium. You may have heard or read about scientists discussing what elements are present in some distant star, and after hearing that, wondered how scientists could know what elements are present in a place no one has ever been. Scientists determine what elements are present in distant stars by analyzing the light that comes from stars and finding the atomic spectrum of elements in that light. If the emission spectrum from the sample contains lines of light that correspond to helium's emission spectrum, then the sample contains helium. Second, and perhaps more importantly, the existence of atomic spectra and the fact that atomic spectra are discontinuous, can tell us a lot about how the atoms of each element are constructed. In general, an element's atomic spectrum results from the interaction between the electrons and protons within an atom of that element. The relationship between atomic spectra and the components of the atom will be the topic of the next lesson.

When scientists use the word continuous, they mean something with no holes, no gaps, and no breaks.

A continuous electromagnetic spectrum contains every wavelength between the wavelength on which the spectrum starts and the wavelength on which the spectrum ends.

A discontinuous electromagnetic spectrum is a spectrum that contains gaps, holes, or breaks in terms of the wavelengths that it contains.

Light from the sun and, in fact, any pure white light source, produces light that contains a continuous spectrum of wavelengths.

White is not a color of light itself, but rather, results when light of every other color is mixed together.

When an electric current or an electric spark is passed through an element, the element will give off a colored glow. This glow is actually composed of light from a discontinuous spectrum that is unique to the each and every element.

We call the discontinuous spectrum produced by passing an electric current through an element the element's atomic spectrum or emission spectrum.

Atomic spectra can be used to identify elements. They also tell us a lot about the nature of matter.

(b) A discontinuous spectrum between 532 nm and 894 nm contains light of every wavelength between 532 nm and 894 nm (including/except for) light with a wavelength of 650 nm.

What is another name for an atomic spectrum?

When an electric current is passed through neon, it glows __________.

When an electric spark is passed through argon, it glows _________.

When an electric current is passed through helium, it glows _________.

LEDs, or light emitting diodes, produce light by passing an electric current through a mixture of different atoms (or molecules) and then using their combined emission spectra to light up a room, or a string of Christmas tree lights. Why are white LEDs difficult and expensive to make?

A unique, discontinuous spectrum emitted by an element when an electric current is passed through a sample of that element.

continuous electromagnetic spectrum

A spectrum that contains every possible wavelength of light between the wavelength at the beginning of the list and the wavelength at the end. In the visible range of light, it is a spectrum which contains every possible color between the color at the beginning of the list and the color at the end.

discontinuous electromagnetic spectrum

A spectrum that includes some, but not all of the wavelengths in the specified range. In the visible spectrum there are gaps or missing colors.